R. Dula

Oxidation of aromatic hydrocarbons with hydrogen peroxide over Zn,Cu,Al-layered double hydroxides

Abstract Layered double hydroxides (LDHs) with general formula [(Zn1−yCuy)1−xAlx(OH)2](NO3)x·nH2O (x≅0.33, 0.6 ≤y≤1.0, n≅0.5) have been synthesized and characterized by XRD, chemical analysis, and ESR. According to ESR, copper ions exist both as isolated species and as clusters, their relative contribution depending on the overall Cu content. The tendency to form clusters is observed even at a very low absolute Cu concentration. The materials are active in the low temperature liquid phase oxidation of toluene, o-, m-, and p-xylene with hydrogen peroxide. The catalytic reaction involves both the side chain oxidation and the hydroxylation/oxidation of the aromatic ring. The catalytically active centres are formed by isolated copper species. The clustered copper ions are inactive in the oxidation reaction but accelerate the non-productive decomposition of hydrogen peroxide. The catalytic performance is discussed in terms of competition between the process of H2O2 decomposition and its consumption in the oxidation reaction.

Layered double hydroxide-derived vanadium catalysts for oxidative dehydrogenation of propane: Influence of interlayer-doping versus layer-doping

Abstract Mixed-oxide vanadium catalysts for oxidative dehydrogenation (ODH) of propane have been prepared by thermal decomposition of Mg, Al-layered double hydroxides (LDHs) containing vanadium either in the brucite layer or in the interlayer. The materials have been characterised by XRD, ICP-AES chemical analysis, XPS, BET and ESR. The catalytic performance of the samples depended on the manner of incorporation of the vanadium into the LDH structure. The sample obtained from interlayer-doped precursor was more active and more selective than mixed oxides obtained from layer-doped LDHs. The difference in the catalytic properties was attributed to the different magnesium vanadates nucleating in the calcined samples, the pyrovanadate formed from the interlayer-doped LDH giving better performance than ortho-vanadate crystallising from the layer-doped precursor. It has been suggested that one of the factors contributing to the difference in the behaviour of both types of catalysts might be the difference in the covalency of VO in-plane bonds around the reduced V centres.

ESR study of the thermal decomposition of V-containing layered double hydroxides

Abstract Heat treatment induced transformations of the ESR signals present in the layered double hydroxides (LDHs) containing vanadium (introduced either as an interlayer decavanadate anion or as a layer-forming cation) are investigated, with particular stress on the changes in the content, electronic structure and symmetry of the vanadyl species created. The differences in the catalytic activity in the oxidative dehydrogenation of propane of the mixed oxide materials resulting from the decomposition of the investigated LDHs are related to the differences in the degree of in-plane π-covalent bonding observed for V(IV) centres present in these materials.

Pretreatment-induced redox processes in molybdena-alumina metathesis catalysts

The ESR investigation of MoO3/Al2O3 metathesis catalysts was carried out and the results interpreted in relation to the previous XPS study [1]. The increase of the Mo(V) signal upon treatment with argon was attributed to the oxidation of ESR-silent Mo(IV) centers. The enhancement of Mo(V) signal upon exposure to propene at 303 K was assigned to the reduction of Mo(VI).

Preparation, physicochemical characterization and catalytic properties of vanadium-doped alumina- and titania-pillared montmorillonites

Summary Procedures leading to preparation of vanadium-doped alumina-and/or titania-pillaredmontmorillonites are described and physicochemical characterization (chemical analysis, XRD, BET, ESR) of the products is provided. Results show that introduction of vanadium into the pillared montmorillonites leads to a rigid association of the dopant with pillars, irrespective of the method of preparation. The mode of vanadyl attachment in aluminapillared samples does not depend on the mode of preparation, while in titania-pillared montmorillonite it does. Certain degree of delocalization of the unpaired electron into ligands and increased in-plane π-covalent bonding is observed for vanadyl ions present in the co-pillared (V-Ti)-PILC samples which also show particularly high activity in catalytic ammoxidation of m-xylene to nitrile product, as monitored by IR. A hypothesis is advanced that this effect is due to the unique character of vanadyl species present in these catalysts.

Vanadium-doped titama-pillared montmorillonites as catalysts for the oxidative dehydrogenation of propane

V-containing titania pillared montmorillomtes have been prepared and characterised with XRD, ESR, BET and chemical analysis. Their performance in the oxidative dehydrogenation of propane depends on the manner of doping. Co-pillarmg gives catalysts of little V content which are less active but more selective than those containing more vanadium and prepared by means of cation exchange. The latter ensure highest yields of propene. The increased selectivity of the co-pillared sample is tentatively assigned to the presence of V centres of higher π-covalency within bonds between vanadium and bridging oxygens.

Composites of Laponite and Cu–Mn Hopcalite-Related Mixed Oxides Prepared from Inverse Microemulsions as Catalysts for Total Oxidation of Toluene

Composites of Laponite and Cu–Mn hopcalite-related mixed oxides, prepared from hydrotalcite-like (Htlc) precursors obtained in inverse microemulsions, were synthesized and characterized with XRF, XRD, SEM, TEM, H2 temperature-programmed reduction (TPR), and N2 adsorption/desorption at −196 °C. The Htlc precursors were precipitated either with NaOH or tetrabutylammonium hydroxide (TBAOH). Al was used as an element facilitating Htlc structure formation, and Ce and/or Zr were added as promoters. The composites calcined at 600 °C are mesoporous structures with similar textural characteristics. The copper–manganite spinel phases formed from the TBAOH-precipitated precursors are less crystalline and more susceptible to reduction than the counterparts obtained from the precursors synthesized with NaOH. The Cu–Mn-based composites are active in the combustion of toluene, and their performance improves further upon the addition of promoters in the following order: Ce < Zr < Zr + Ce. The composites whose active phases are prepared with TBAOH are more active than their counterparts obtained with the use of the precursors precipitated with NaOH, due to the better reducibility of the less crystalline mixed oxide active phase.